After copying and pasting I cannot deselect an area

Hi All,
Sorry, really inexperienced noob here. I have a pic where there's a horrible sign post in the foreground which I didn't spot when i took the pic.
I want to get rid of it, so i'm copying various parts of the background, and pasting them over the signpost. unfortunately, after pasting a section, i can't seem to deselect the newly pasted section. I've had a google and many forums are telling me to go Select -> None, but this is greyed out. I can save the picture, close it, and reopen in GIMP, but i dont want to do this everytime I paste.

This is because "Paste" creates a "floating selection" which is some sort of temporary layer.
Open the Layers dialog (Ctrl-L or Windows/Dockable Dialog/Layers). The top of the list is the "floating selection". Right-click on it and "Anchor" it (this merges it with the layer below), or use "New layer", which will put it in its own layer, so you can move it or make it partially transparent independently.

Otherwise, the best way to get rid of unwanted details in a picture is the "Clone" tool (or better the "'Heal" tools in some cases. You can even clone stuff from another picture.